Pulse shaping filters are filters used in digital communications. In digital telecommunications, for instance, pulse shaping is the process of altering a waveform of transmitted pulses. One purpose of a pulse shaping filter is to change the transmitted signal to suit better communication channels by limiting the effective bandwidth of the transmission. In essence, by filtering the transmitted pulses, the interference caused by the channel can be controlled and/or reduced.
In addition, pulse shaping is used to increase transmission data rates without increasing the bandwidth or the bit error rate of the signal. Preferably, the pulse shape used for transmission has a low bandwidth and no inter-symbol interference (ISI). A sinc function includes both of these properties and thus can significantly increase spectral efficiency. A system using sinc functionality for pulse shaping can be susceptible to timing jitter—thus is not practical—and is challenging to precisely implement. A rectangular wave, on the other hand, is not sensitive to timing jitter, but requires a large bandwidth. Commonly-used pulse shapes, such as raised cosine, can provide a compromise between the two extremes of the sinc function and the rectangular wave.
With the advent of multi-media streaming and other high data throughput applications, the required speed of operation of pulse shaping filters is growing. Conventional finite impulse response (FIR) implementations are no longer sufficient to meet these requirements.
For example, FIG. 1 illustrates a block diagram of a conventional pulse shaping filter 10 having an upsampling factor of 2, which is capable of operating up to a few mega bits per second (Mbps). The pulse shaping filter 10 of FIG. 1 includes a delay system 20, a multiplier system 40, and an adding system 60.
The pulse shaping filter 10 can receive an input signal 12. Upon receipt, the input signal 12 is delayed by a predetermined number of delays blocks with delays of half the symbol rate 24 of the delay system 20. As illustrated in FIG. 1, the delays 24 are chained together to make up the delay system 20. Each tap 22, after going through the delay 24, is output to a multiplier 42 of the multiplier system 40. Each multiplier 42 receives both a coefficient (i.e., ci) and a signal 26 from the different delayed versions of the input signal 12. The multiplier 42 multiples the coefficient (i.e., ci) by the signal 26 received from the delay 24. Each of the outputs 44 of the multiplier 42 are input into the adding system 60, which then sums for an output 16 of the PSF 10.
Considering that a 13-tap raised cosine filter for a 2 giga bits per second (Gbps) wireless transmission having an upsampling factor of two the filter has to operate at 4 Giga Hertz (GHz). Unfortunately, a digital FIR implementation using multipliers and adders is not possible; it requires challenging multiplication which takes excessive power and due to the mathematics necessary for calculations will delay processing. Alternate filter implementations are required which can support such high data-rates.